How much does the soul weigh?

An everlasting soul is a powerful concept; it's the central feature of many religions and a deeply comforting belief in the face of loss. 

Perhaps that's why some have been dissatisfied with leaving matters of the soul to faith, instead turning to science in attempts to prove the soul exists. If you've ever heard that the soul weighs 21 grams — or seen the 2003 film “21 grams” alluding to this fact — you've heard the results of one of these rather unusual experiments.

So how much does the soul really weigh? Well, the bad news is that, of course, no one can say. Science can't prove that the soul exists, and scientists can't weigh it. But the bizarre story of one doctor's attempt to do just that is worth hanging around for. 

The story starts at the turn of the last century in Dorchester, a neighborhood in Boston. A reputable physician named Duncan MacDougall had a bee in his bonnet: If humans had souls, he thought, those souls must take up space. And if souls take up space, well, they must weigh something — right?

Weighing the soul

There was just one way to find out, MacDougall reasoned. "Since … the substance considered in our hypothesis is linked organically with the body until death takes place, it appears to me more reasonable to think that it must be some form of gravitative matter, and therefore capable of being detected at death by weighing a human being in the act of death," he wrote in the scientific paper he would eventually publish in 1907 about this effort. 

MacDougall teamed up with Dorchester's Consumptives' Home, a charitable hospital for late-stage tuberculosis, which at that time was incurable. MacDougall built a large scale, capable of holding a cot and a dying tuberculosis patient. Tuberculosis was a convenient disease for this experiment, MacDougall explained in his paper, because patients died in "great exhaustion" and without any movement that would jiggle his scale. 

MacDougall's first patient, a man, died on April 10, 1901, with a sudden drop in the scale of 0.75 ounce (21.2 grams). And in that moment, the legend was born. It didn't matter much that MacDougall's next patient lost 0.5 ounce (14 grams) 15 minutes after he stopped breathing, or that his third case showed an inexplicable two-step loss of 0.5 ounce and then 1 ounce (28.3 g) a minute later. 

MacDougall threw out Case 4, a woman dying of diabetes, because the scale wasn't well calibrated, in part due to a "good deal of interference by people opposed to our work," which raises a few questions that MacDougall did not seem eager to answer in his write-up. Case 5 lost 0.375 ounce (10.6 grams), but the scale malfunctioned afterward, raising questions about those numbers, too. Case 6 got thrown out because the patient died while MacDougall was still adjusting his scale. 

MacDougall then repeated the experiments on 15 dogs and found no loss of weight — indicating, to his mind, that all dogs definitely do not go to heaven. 

MacDougall reported his results in 1907 in the journal American Medicine and the Journal of the American Society for Psychical Research. He also snagged a write-up in The New York Times. 

Unanswerable questions

MacDougall's study had a minuscule sample size, and his results were all over the place, so even at the time, it cast the notion that he measured the soul into serious doubt. To MacDougall's credit, he admitted that more measurements were needed to confirm that the soul had weight. That hasn't happened — in part for ethical reasons, and in part because the experiments are a bit … kooky. A rancher in Oregon did attempt to replicate the soul-weighing experiment with a dozen sheep in early 2000, according to Mary Roach's book "Spook: Science Tackles the Afterlife" (W. W. Norton & Co., 2005). Most gained between 1 and 7 ounces (30 to 200 grams), though the gains lasted just a few seconds before the sheep returned to their original weights. 

Roach also reported that Dr. Gerry Nahum, a chemical engineer and physician who was at the Duke University School of Medicine at the time, had developed a hypothesis that the soul, or at least the consciousness, must be associated with information, which is equivalent to a certain amount of energy. Because the equation E = mc ^2 dictates that energy equals mass times the speed of light squared (thanks, Einstein), this energy could, essentially, be weighed with sensitive enough electromagnetic instruments. As of 2007, Nahum had not gotten funding for experiments that would prove whether he was right. He now works for Bayer Pharmaceuticals. (Roach wrote that Nahum did not hope to pull a MacDougall and do his tests on humans. Instead, he was considering leeches as subjects.) 

The bottom line is that science has not remotely determined the weight of the soul, nor whether the soul exists at all. Chances are, this question will be left to the religious realm. 

Originally published on Live Science on Dec. 01, 2012, and rewritten on July 25, 2022.